Vestibular schwannomas (VSs) develop from the Schwann cells (SCs) of the vestibular nerve. They cause significant morbidity including deafness, tinnitus, facial paralysis, imbalance, and brainstem compression. Microsurgical removal and stereotactic radiosurgery/stereotactic radiotherapy (SRS/SRT) constitute the only current treatments for VSs and often result in deafness, tinnitus, facial paralysis and imbalance themselves. Further, some patients that require treatment are not good candidates for either microsurgery or SRS/SRT. Development of effective alternative therapies will be of great benefit to those patients unsuitable for current therapeutic options. Understanding the factors that contribute to VS tumorigenesis and radiosensitivity will help identify potential therapeutic targets. Merlin, the defective gene product in VSs, regulates the activity of kinases implicated in tumorigenesis including extracellular regulated kinases (ERKs), phosphatidylinositol-3-kinase (PI3-K)/Akt, and c-Jun N-terminal kinases (JNKs). Preliminary data demonstrate activation of ERKs and PI-3K/Akt in VS cells leading to increased proliferation and activation of JNK promoting cell proliferation and survival. Inhibition of JNK activity increases accumulation of reactive oxygen species (ROS), including mitochondrial superoxides. Further ROS scavengers prevent apoptosis in VS cells with suppressed JNK, implying that the prosurvival effects of JNK may be due to its ability to reduce oxidative stress. Conversely, JNK activity appears to promote apoptosis in denervated SCs. Finally, VS cells appear highly resistant to irradiation (IR). These observations lead to the hypothesis that loss of merlin function in VS cells results in persistent JNK activation which, in turn, suppresses accumulation of ROS, promotes cell survival, and confers radioresistance. The goal of these studies is to test this hypothesis in primary human VS cell cultures and human VS xenografts implanted in nude mice. The first aim is to determine whether merlin inhibits JNK activity in human VS cells and in SCs and identify the upstream signaling leading to JNK activation. This will be addressed by replacing functional merlin into VS cells using viral-mediated gene transfer and determining if JNK activity is consequently suppressed. Parallel studies will ask if SCs derived from transgenic mice lacking functional merlin demonstrate increased JNK activity. The second aim will use primary human VS cultures and orthotopic xenografts in nude mice to determine the extent to which persistent JNK activity promotes VS growth by increasing cell proliferation and survival. The third aim seeks to determine if suppression of specific reactive oxygen species (ROS) by JNK contributes to VS cell survival and the final aim seeks to determine whether JNK inhibitors potentiate the ability of 3-irradiation (IR) to induce VS cell apoptosis and reduce proliferation. The results of these studies will provide insights into the fundamental mechanisms contributing to VS cell growth and radiosensitivity and will likely contribute to the development of novel therapies for VSs.